Projection Screen, in Particular for Video Projection Reducing the Moire Effect

ABSTRACT

A projection screen comprises a film having a projection side and provided with a number of passages adapted for permitting emitted sound waves to pass through said film. The projection screen is characterized in that these passages are arranged in such a manner that, if alignments can be detected, at least the principal ones have an angle that is not zero with regard to the vertical and horizontal edges of the screen.

The invention, relates to a projection screen, intended in particularfor video applications. To be more precise, it is directed to a screenthat is permeable to sound waves.

The advantages of screens that are permeable to sound waves are known,in that they enable sound sources to be placed behind the screen,thereby achieving a perceptible cohesion between the projected image andthe sound (whether the image is front-projected or back-projected).

Those advantages are described in particular in the document FR 2810122(CONGARD) and its counterparts EP-1162499 and U.S. Pat. No. 6,552,847.

Disposing the sound sources, in practice loudspeakers, behind a screenin this way is routine in movie theaters, where the screen isconventionally perforated with small holes intended to enable sound topass through.

In video applications, and in domestic applications in particular, thevisibility of the holes is often increased by the proximity of thespectators to the screen. The size and spacing of the holes areconventionally determined by the tools used to produce them,independently of the size of the screen.

Certain manufacturers have recently developed tools for reducing thesize and spacing of the perforations whilst preserving a substantiallyconstant ratio of the perforated area to the total area of the screen.

Screens using “woven” technology are also used for these applications.

Most recent video projectors are of the fixed pixel matrix type, usingthe digital light processor (DLP) or liquid crystal display (LCD)technique, for example. These fixed pixel projectors divide the videoimage into individual elements each of which is assigned a color and avalue.

Projectors of the above kind project an image in which the rows ofpixels are visible under certain conditions, especially if the image isviewed from a distance less than the recommended distance. The rows ofpixels are in particular apparent as orthogonal alignments of the spacesbetween pixels parallel to the edges of the image.

In practice, it is found that the order of magnitude of the spacingbetween the holes in perforated video screens and the dimensions of aprojected pixel are often similar. This results in visible interferencebetween two periodic alignments, producing unwanted Moiré effects.Disposing the holes in a quincunx arrangement to reduce their visibilityhas already been proposed, but has achieved mixed results.

One makeshift solution is to adjust the distance between the projectorand the projection screen, which varies the size of the projected imageand therefore the size of the pixels, to find positions that generatethe least Moiré effect. The results are mixed and do not guarantee thatchanging projector, for example for one with a higher definition andtherefore a different pixel size, will not generate Moiré phenomenaagain.

These unwanted effects are also encountered with woven screens. When thestructure of the canvas causes spaces or projected shadow effects toappear, the periodic alignment thereof generates the same unwanted Moiréeffects as in perforated video screens.

Technologies developed with the object of obtaining a good compromisebetween image quality and permeability to sound waves that do not takeaccount of the risk of the effects of resonance with the fixed matrixtype technology (Moiré effects), have therefore proven to be incapableof achieving really good results.

In the situation where the flatness of the screen and/or itsorthogonality to the projection axis are not perfect, which is the casein many installations in practice, the Moiré effects are much worse,whence even worse results.

For these reasons, screens that are permeable to sound are at presentconsidered to be problematical in terms of compatibility with fixedpixel projectors, i.e. in practice with the great majority of projectorsavailable, in particular for video projection.

The object of the present invention is to propose a screen that ispermeable to sound waves and that produces little or no visible opticalinterference when it is used with a fixed pixel projector, whilst at thesame time being simple and inexpensive, and which can be stretched on aframe or of the roll-up type.

To this end the invention proposes a projection screen including a sheethaving a projection face and provided with a plurality of passagesadapted to allow sound waves to pass through the sheet, characterized inthat the disposition of the passages of said plurality is such that, ifalignments thereof can be detected, at least the main alignments have anon-zero inclination relative to the vertical and horizontal edges ofthe screen.

In the context of the invention, if there are any alignments, the mainones are those that are the most dense, i.e. that have the most passagesper unit length.

It has become apparent that this absence of parallelism at the edges ofthe screen (and thus the existence of this inclination) cancels out orgreatly reduces unwanted Moiré effects by reducing the risk ofcoincidence between the vertical and horizontal alignments of theprojected image pixel structures and the projection screen passage orweft structure, since the sides of the screen are in practice parallelto the edges of the projected image.

The non-zero inclination is preferably greater than 5° or even greaterthan 10°, for example from 5° to 25°.

If the passages or, where applicable, the weft of the sheet form manyalignments, the latter are inclined with the result that none of them,or as few of them as possible, are parallel to the vertical andhorizontal edges of the screen.

The sheet is preferably made from a plurality of threads or by a methodsimulating the appearance of an array of threads (for example by moldingplastics materials or by cutting out or “expanding” a sheet of solidmaterial), but may also be made from a solid sheet perforated with smallholes.

The risk of interference is further reduced if the plurality of passagesor the weft features substantial geometry or orientation variations.

Because of these substantial geometry or orientation variations, when alight beam is directed toward the projection face, there are differentphysical interactions from one passage to another, which further reducesMoiré effects.

These variations may exist at the level of the passages consideredindividually, for example by virtue of the use of threads that are notsmooth, for example bouclé threads (the contours of the passages arethen irregular, including lengthwise) and/or by virtue of a treatment ofthe sheet, such as a dressing treatment (the contours of the passagesare modified relative to their initial shape under the dressing). Thusthe sheet is advantageously a crepe (woven or knitted, often using abouclé and/or undulating thread, and being subjected to a dressingtreatment).

There may also be variations from one passage to another.

Thus the variations may result from a difference of shape or orientation(in the plane of the sheet) from one passage to another: thus thepassages may have an elongate shape, for example, with a greaterdimension whose orientation varies from one hole to another.Accordingly, even if the locations of the passages form a periodicarray, those of the passages that are strictly identical and thereforereact identically to the incident light beams form an array that is muchlarger than that of the locations of the passages; without wishing tolimit the invention by virtue of this interpretation, this would seem tobe beneficial for reducing the risk of Moiré effects. These variationsmay be fluctuations, where appropriate periodic fluctuations.

These variations may also result from different orientations of thepassages relative to the plane of the sheet, for example by virtue ofthe existence of surface patterns in one or more dimensions; one exampleof a multidimensional pattern is a lozenge or rectangle pattern, and oneexample of a unidimensional pattern is a linear array of undulations,for example a set of ribs. Accordingly, even if the passages are formedat locations forming a periodic array, the surface undulations mean thatthe passages react differently to light beams.

The sheet may advantageously be made of tweed, which yields passages (ofdifferent shapes) that are substantially aligned in more than onedirection inclined to the plane of the sheet, for example with a chevrondisposition (the woven threads follow a zig-zag path).

Another form of weaving is weaving individual threads in one directionand double threads in another direction, which combines the advantagesof different passage shapes with those of surface undulations (lines inrelief, in a direction inclined to each of the global directions of thewoven threads). Note that this kind of weave is similar to that of denimjeans.

It might be emphasized that, in practice, the passages of a sheet of theinvention are not very visible, if at all, to a spectator situated atthe usual viewing distance and in the usual viewing direction (typicallyone and a half times the width of the screen, even in a domesticapplication, and thus with small screens).

The sheet may be produced from textile threads, but also from plasticsmaterial threads, for example polyester threads, advantageously bouclépolyester threads. The threads may be assembled by weaving or by anyother process, for example knitting. The sheet may also be non-woven.

Preferred features of the invention, which may be used individually orin combination, include:

-   -   said sheet is solid but perforated;    -   said sheet is woven;    -   said sheet is woven with different numbers of warp threads and        weft threads;    -   the spaces between the threads do not exceed 0.3 mm;    -   the projection face of the sheet has ribs inclined to the        general directions of the warp and weft threads;    -   a coated polyvinyl chloride thread is used for weaving;    -   said sheet is of knitted thread;    -   the thread is a bouclé thread;    -   the thread is a polyester thread;    -   no weft is visible;    -   said surface is of crepe;    -   the sheet is woven and is of the satin type;    -   the sheet is woven and is of the tweed type;    -   the projection screen includes a second layer that is permeable        to sound waves, substantially superposed on the first layer and        placed behind said first layer relative to the light flux from        the projector; and    -   a sound source is disposed behind said screen and a light source        is disposed in front of it.

The invention also proposes an installation including a screen of thetype cited above, a sound source disposed behind that screen and a lightsource disposed in front of it.

Objects, features and advantages of the invention emerge from thefollowing description, which is given by way of non-limitingillustrative example and with reference to the appended drawings, inwhich:

FIG. 1 is an overall perspective view of a projection screen that ispermeable to sound placed in front of a sound source,

FIG. 2 is an enlarged front view of a prior art screen material,

FIG. 3 is a front view of a conventional canvas type woven material,

FIG. 4 is a front view of a woven material constituting, afterinclination in its plane, a first embodiment of a screen of theinvention,

FIG. 5 is a front view of a first face of a woven material constitutinga second embodiment of a screen of the invention when illuminated at anangle of about 45° to the normal to the plane of the screen,

FIG. 6 is a view of the second (rear) face of the same woven materialwith the same illumination,

FIG. 7 is a front view of a woven material identical to that of FIG. 5when illuminated in a direction normal to the plane of the screen andpassing through its center,

FIG. 8 is a front view of a woven material constituting a thirdembodiment of a screen of the invention,

FIG. 9 is a diagram indicating a preferred orientation relative to theedges of the screen of a material like those of FIGS. 2, 3 and 4,

FIG. 10 is a diagram of a perforated screen indicating a preferredorientation of the alignments of perforations relative to the edges ofthe screen, and

FIG. 11 is a diagram of a more complex (quincunx arrangement) perforatedscreen, indicating multiple alignments relative to the edges of thescreen.

FIG. 1 is a diagram representing a projection screen 4 that is permeableto sound waves. An image 5 is projected onto the screen 4, representinga person speaking. A sound source 6 (represented in dashed outline) isplaced behind the screen 4, substantially at its center. The soundsource 6 reproduces a sound recording made of the person speaking whenrecording the image, usually in video. Here the image is front-projected(by a projector 7), but it could instead be back-projected.

The picture and sound recordings are synchronized.

This conventional arrangement is intended to reproduce sound and picturesimultaneously and coherently. Placing the sound source at any otherplace would eliminate this cohesion, which would degrade the overallquality of the reproduction of the synchronized sound and picturerecordings.

The disposition represented in FIG. 1 is conventional, and isencountered in most movie theaters, often enhanced by the presence oftwo further (right-hand and left-hand) sound sources (not shown) forstereophonic reproduction of sound.

However, the disposition represented in FIG. 1 is possible only if thescreen is made of a material that is permeable to sound waves.

In a prior art configuration shown diagrammatically in FIG. 2, amaterial of this kind generally consists of a thin (0.2 to 0.7 mm) sheetof plastics material, generally PVC. This sheet 10 is perforated with aplurality of holes 10′ homogeneously distributed over the whole of thesurface of the screen at a regular pitch of 8 to 15 mm. The holes 10′have a diameter from 0.8 to 1.5 mm. These holes are provided in thematerial forming the screen to render it permeable to sound, thispermeability to sound not being inherent to the nature of the material.Screens of the above kind are generally encountered in movie theaters.

Although screens of the above kind are generally satisfactory in movietheater installations, this is not the case in domestic video projectioninstallations. Recent developments in video applications, in particularfor domestic use, have highlighted the limitations of this kind ofscreen. In such applications, the spectators are closer to the screenthan in a movie theater, the screens themselves being smaller (with abase length from 180 to 320 cm in most cases). Holes of the above orderof magnitude therefore become visible at the usual viewing distances (3to 10 m).

Screens made from perforated plastics materials have been improved byusing holes of smaller diameter (typically 0.5 mm) with a smallerspacing (typically 5 mm) to preserve acceptable permeability to sound.Moreover, in an attempt to reduce the undesirable visibility of theseholes, it has been proposed to dispose them in a quincunx arrangement.Improved screens of the above type with base dimensions from 2 to 3.5 moffer slightly better video image quality than can be obtained withconventional perforated movie theater screens, but without preventingMoiré phenomena (see below).

What is more, their permeability to sound is still low, and can beimproved only to the detriment of image quality (a greater density ofperforations in the projection surface reduces the reflecting surfacearea).

The development of fixed pixel video projectors, which offer manyadvantages over earlier scanning type projectors, has given rise to aproblem of compatibility with perforated screens.

Fixed pixel projectors have a particular number of pixels, disposed in amatrix made up of rows and columns. The divisions between the rows andcolumns of pixels are monochrome areas of constant value, and thereforeform visible lines projected onto the screen.

These lines form an orthogonal grid along the rows and columns of thematrix of pixels, which is not particularly troublesome in itself if theviewing distance is sufficient (a viewing distance is generallyconsidered to be sufficient if the dimension of a projected pixel isless than or equal to the limit of visibility, which is from 1 to 5minutes of arc, depending on the person).

Superposition of the projected grid and the array formed by holes inperforated canvas, disposed in regular horizontal and verticalalignments, creates a form of optical interference known as the Moiréeffect, producing lines both wider and farther apart than the pitch ofthe projected grids or of the alignments of holes. These Moiré effectsare highly visible and constitute unwanted picture artefacts.

Moreover, if the plane defined by the projection surface is not exactlyperpendicular to the projection axis, or if its flatness is imperfect,the visible Moiré lines are no longer parallel to the lines of the gridsthat produce them, which makes them even more visible.

What is more, when projecting moving video images, alternating darksscenes, masking the projected grid, and light scenes, makes the Moiréeffect intermittent and therefore even more visible, as there is noaccommodation.

This phenomenon has led many screen users to conclude that perforatedscreens are not readily compatible with fixed pixel projectors.

FIG. 3 is a diagram representing a woven screen 11 offering goodpermeability to sound, but generally to the detriment of video imagereproduction quality. The weft threads 12 are sufficiently spaced toproduce interstitial passages 13 offering good permeability to sound.Woven screens of this type have a visible grid appearance, liable tointerfere optically with the grids projected by fixed pixel videoprojectors. Note that the weave is very simple, limited to the crossingof individual threads in two perpendicular directions; this particularlysimple type of weave is often referred to as “canvas”. The threads havea diameter of 0.5 mm, for example.

The subsequent figures correspond to various ways of arranging textileor plastics materials threads that are knitted, woven, non-woven orassembled in any other manner, that, in accordance with the invention,significantly reduce Moiré effects.

These arrangements have the common feature of a sheet having aprojection face and a plurality of passages adapted to allow sound wavesto pass through the sheet, the disposition of the plurality of passagesbeing such that, if alignments can be detected therein, at least themain alignments have a non-zero inclination with respect to the edges ofthe screen. Moreover, several of the plurality of passages havesignificant geometry or orientation variations.

In a first embodiment of the invention, as shown in FIG. 4, theprojection screen is made from a woven sheet 17 so that the warp threads18 and the weft threads 19 appear contiguous when they are viewed in adirection approximately normal to the plane of the screen.

The threads being interleaved in a simple pattern, namely that ofcanvas, but being paired, there are significant differences of shape ororientation between the various passages, according to whether they arebetween the two warp threads or not, whilst being situated between thetwo weft threads or not.

The threads themselves have a diameter from 0.02 to 0.2 mm.

Accordingly, the pitch of a (where applicable visible) array of thiskind of woven material is from 10 to 100 times finer than that of theprojection of an SXGA matrix (1200×1024 pixels, at present the mostusual resolution of projectors for domestic use), projected onto ascreen with a base of 2.40 m.

This pitch remains of a very different order of magnitude compared tothat of matrices projected by projectors offering higher resolutionsthat may be envisaged in the near future.

The screen is obtained by inclining the sheet at an angle α by rotationin its plane, so that the warp and weft threads form any non-zero angleto the edges of the screen. This angle α is advantageously from 5 to 25°(see FIG. 9).

To this end, the rectangular piece constituting the screen may be cutout from the woven fabric on the bias, at this bias angle (see thechain-dotted lines in FIG. 4).

This kind of inclination may be applied to any passage structure, even aprior art structure, in order to depart as far as possible from aperiodic array of a projected pixel structure the edges whereof are inpractice parallel to the main alignments of the array.

In practice, it is found that the projection of a video image onto ascreen with a base length from 1.8 to 3 m in accordance with this firstembodiment of the invention does not produce any visible Moiré effectwith existing projectors.

It may be noted that, because of the geometry of the particular passagesof this embodiment, the progress of the sound wave through the wovenmaterial occurs, throughout the audible frequency band, by diffractionaround the threads, since the latter in practice have a diameter lessthan the wavelength of the maximum audible frequency (17.15 mm).

A second embodiment of a screen of the invention, as represented inFIGS. 5 to 7, uses a projection surface 25 consisting of a satin typewoven material, sometimes known as tweed, similar to the weaving mode ofdenim jeans.

This kind of woven material has two faces of different appearance, aface A being represented in FIG. 5 and a face B in FIG. 6.

Generally speaking, this sheet has a double structure, one aconventional orthogonal warp and weft structure and the other a biasstructure, at an angle of the order of 30°, for example.

FIGS. 5 and 6 show the woven fabric when illuminated in a directionglobally oblique to the plane of the material. This kind of illuminationhighlights the relief of the woven material, which overall has obliquerows 26 (FIG. 6A) in the form of grooves, bordered by raised portions27. The height of the raised portions 27 relative to the grooves 26 issubstantially equal to the diameter of the thread.

Through shadowing, oblique illumination accentuates the interstices 28between the threads, which do not correspond to the interstices 29visible on the face B of the woven material.

FIG. 7 shows the face A of the woven material used in this embodiment ofthe invention when illuminated in a direction substantially normal tothe plane of the woven material. Under this illumination, whichcorresponds to that produced by a video projector in use, the passages28′ between the threads are not (or not very) apparent, because of theirdifferent positions on faces A and B.

It follows from the above that, independently of the inclination of theindividual threads, the passages intended for the sound waves to passthrough the sheet have alignments that are inclined with respect to theedges (see the chain-dotted lines). These passages advantageously alsohave an orientation and a geometry that vary across the sheet and fromone passage to an adjacent passage.

Thus the woven material is practically opaque to light and, in videoprojection, has no structure parallel to the edges that is clearlyvisible and liable to interfere optically to an unacceptable degree witha grid projected onto the screen.

Apart from the favorable effect of inclining the alignments of passagesthat may be detected, one advantage of this embodiment is that itachieves at least approximate concealment of the passages 28′ betweenthe threads, nevertheless enabling the latter to have a dimensionsufficient to optimize the permeability of the screen 25 to air andtherefore to sound waves.

The thread selected is preferably polyvinyl chloride with a fibrouscore, for example a glass fiber, carbon fiber or kevlar fiber core,although this is not limiting on the invention.

The diameter of the thread is preferably from 0.1 mm to 0.17 mm,although this is also not limiting on the invention.

The distances between the threads preferably do not exceed 0.3 mm.

As in the preceding embodiment, the sheet is advantageously rotated inits plane so that the warp and weft threads are at any non-zero angle tothe edges of the screen.

FIG. 8 is a diagram showing another embodiment with, on the front faceof the woven material concerned, individual weft threads that areinterleaved with pairs of warp threads (each time passing in front ofthree such pairs, with an offset of one pair on passing from one weftthread to the next). The rear face of this kind of woven material isanalogous to that represented in FIG. 7, i.e. the pairs of warp threadsare doubled up on the rear face.

This type of weave also features passages 30 disposed in an array, butwith a significant inclination of the rows of passages to the edges (seethe chain-dotted lines).

Accordingly, even if it cannot really be said that the passages are notvisible, any alignments that may be detected have significantinclinations to the edges, thanks to which there is a significantreduction in the risk of formation of Moiré patterns.

Another embodiment of the screen uses a crepe or pique type wovenmaterial, preferably knitted, woven or non-woven.

A crepe or pique woven material knitted from bouclé thread with adiameter from 0.02 mm to 0.1 mm is preferably selected.

A synthetic, polyester or acrylic thread is preferably selected.

Although obtained with a weave that may be very simple, a woven materialof the above kind, because of the crepe and/or bouclé nature of thethread that produce an interleaving effect in the passages, has theparticular feature of offering no visible periodic structure, andtherefore no alignment of passages that is readily detectable. Whateverthe size of the image and the resolution of the projector, no opticalinterference occurs when a grid of pixels is projected.

A woven material generally has a resistance to penetration of airrelated to its density. Reducing its density improves its permeabilityto air, and therefore to sound waves, but reduces its opacity to light,which degrades the projected image reproduction quality.

It has been found that a screen of woven material of the crepe or piquetype offers optimum opacity and optimum permeability to air for adensity from 150 to 220 g/m².

Another embodiment of the invention creates passages in a solid sheet inthe form of slots formed on the bias relative to the plane of the sheet,so that in the plane of the sheet they are not seen as perforations.

It follows from the foregoing that a screen of the invention may beproduced by pivoting in its plane a sheet having any structure made upof passages so that the structure is inclined to the edges of thescreen, preferably at an angle from 5 to 25° (see FIG. 9).

Thus FIG. 10 represents a screen whose edges have been cut on the biasfrom a sheet of the FIG. 2 kind; it is easy to identify therein thesquare mesh array from FIG. 2 (the passages 10″ correspond to thepassages 10′ in FIG. 2).

FIG. 11 shows an array of passages 40 that may be defined in variousways. Considering the two orientations inclined at the angle α to thehorizontal and vertical edges of the screen, a square mesh array isseen. On the other hand, considering the two orientations inclined atthe angle β to the same horizontal and vertical edges of the screen, aquincunx array is seen. Finally, if an inclination direction a relativeto one side of the screen is considered in combination with aninclination direction β relative to another side of that screen, anon-rectangular parallelepiped mesh network is seen. Furthermore, manyother alignments may be identified, such as that characterized by aninclination γ. According to the invention, at least the main alignments,and thus those which are the most dense, have a non-zero inclinationrelative to the edges of the screen.

In addition to a sheet of one of the types described above, the screenmay have a second layer substantially superimposed on the first,permeable to sound waves and placed behind said first layer relative tothe light flux from the projector.

In all the embodiments of the present invention described, the wovenmaterial used to constitute a screen projection surface is preferablywhite. However, in certain applications, the use of pale grey wovenmaterials may be envisaged, or materials having some other color closeto white, or even different.

1. Projection screen including a sheet having a projection face andprovided with a plurality of passages adapted to allow emitted soundwaves to pass through said sheet, characterized in that the dispositionof the passages of said plurality is such that, if alignments thereofcan be detected, at least the main alignments have a non-zeroinclination relative to the vertical and horizontal edges of the screen.2. Projection screen according to claim 1, characterized in that thisplurality of passages has substantial geometry or orientationvariations.
 3. Projection screen according to claim 1, characterized inthat said sheet is perforated.
 4. Projection screen according to claim1, characterized in that said sheet is woven.
 7. Projection screenaccording to claim 4, characterized in that the sheet has ribs on theprojection face inclined relative to the general directions of the warpand weft threads.
 8. Projection screen according to claim 4,characterized in that weaving utilizes threads coated with polyvinylchloride.
 9. Projection screen according to claim 1, characterized inthat said sheet is of knitted thread.
 10. Projection screen according toclaim 1, characterized in that the thread is a bouclé thread. 11.Projection screen according to claim 10, characterized in that thethread is a polyester thread.
 12. Projection screen according to claim1, characterized in that no weft threads are apparent.
 13. Projectionscreen according to claim 1, characterized in that said surface is acrepe surface.
 14. Projection screen according to claim 1, characterizedin that the sheet is a woven sheet of the satin type.
 15. Projectionscreen according to claim 1, characterized in that the sheet is a wovensheet of the tweed type.
 16. Projection screen according to claim 1,characterized in that it includes a second layer that is permeable tosound waves, substantially superposed on the first layer and placedbehind said first layer relative to the light flux from the projector.17. Projection installation including a screen according to claim 1, asound source disposed behind the screen and a light source disposed infront of it.